Brake control apparatus for vehicle

ABSTRACT

A brake control apparatus includes: a master cylinder that outputs a brake fluid at a master pressure; a master pressure changing device that can change the master pressure irrespective of an operation of a brake pedal; a brake actuator; and a control unit that executes antilock control by reducing a brake pressure of a target wheel. Modes of the antilock control include a normal mode and a pseudo mode. In the pseudo mode, the control unit operates the master pressure changing device such that the master pressure obtains a target value of the brake pressure of the target wheel, and changes the brake pressure of the target wheel in an interlocking manner with the master pressure. When the normal mode is unavailable, the control unit executes the antilock control in the pseudo mode.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2016-156845 filed onAug. 9, 2016 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a brake control apparatus for avehicle. In particular, the present disclosure relates to a brakecontrol apparatus that executes antilock control of a wheel.

2. Description of Related Art

A braking device for a vehicle is disclosed in Japanese PatentApplication Publication No. 2015-136993 (JP 2015-136993 A). The brakingdevice for the vehicle includes a master cylinder, a servo pressuregenerator, and a brake actuator. The servo pressure generator generatesa servo pressure irrespective of an operation of a brake pedal by adriver. The master cylinder is operated on the basis of the servopressure and outputs a brake fluid at a master pressure that correspondsto the servo pressure to the brake actuator. The brake actuatordistributes the brake fluid, which is output from the master cylinder,to a wheel cylinder of each wheel. In addition, the brake actuator canindividually control a pressure of the brake fluid, which is supplied tothe wheel cylinder of each of the wheels.

Antilock control for preventing locking of the wheel during braking hasbeen known. In a related art, the antilock control has been executed byindividually controlling a brake pressure of a target wheel by using thebrake actuator.

SUMMARY

In the cases where the antilock control using the brake actuator isunavailable and thus the execution of the antilock control is abandoned,safety is degraded.

The present disclosure provides a technique capable of executingantilock control even when antilock control using the brake actuator isunavailable.

A first aspect of the present disclosure provides a brake controlapparatus for a vehicle. The brake control apparatus according to thefirst aspect includes: a master cylinder configured to output a brakefluid at a master pressure; a master pressure changing device configuredto change the master pressure irrespective of an operation of a brakepedal; a brake actuator configured to supply the brake fluid output fromthe master cylinder to a wheel cylinder of each of wheels and that cancontrol a brake pressure of the brake fluid supplied to the wheelcylinder; and a control unit configured to execute antilock control ofpreventing locking of a target wheel by reducing the brake pressure ofthe target wheel irrespective of the operation of the brake pedal duringbraking. Modes of the antilock control include a normal mode and apseudo mode. The control unit is configured to: in the normal mode,operate the brake actuator to obtain a target value of the brakepressure of the target wheel; in the pseudo mode, operate the masterpressure changing device such that the master pressure obtains thetarget value, and change the brake pressure of the target wheel in aninterlocking manner with the master pressure; and execute the antilockcontrol in the pseudo mode when the normal mode is unavailable.

According to the above configuration, even when the normal mode usingthe brake actuator is unavailable, the antilock control can be executedin the pseudo mode. Thus, compared to the related art, an opportunityand a period to execute the antilock control are increased, and safetyis improved.

In the first aspect, the brake actuator may include a pump, and thecontrol unit may be configured to execute the antilock control in thepseudo mode when the pump cannot be used.

According to the above configuration, even when the pump provided in thebrake actuator is unavailable, the antilock control can be executed inthe pseudo mode.

In the first aspect, the brake actuator may include: an input node thatreceives the brake fluid output from the master cylinder; a boostervalve that is provided between the input node and the wheel cylinder foreach of the wheels; a reduction valve that is provided between the wheelcylinder and a reservoir for each of the wheels; and a pump that isconfigured to causes the brake fluid to return from the reservoir to theinput node.

In the above configuration, in the pseudo mode, all the wheels may bethe target wheels, and the control unit may be configured to reduce thebrake pressures by opening the booster valves and closing the reductionvalves for all the wheels and reducing the master pressure.

According to the above configuration, all the wheels can be set as thetarget wheels in the pseudo mode. In this case, the antilock control inthe pseudo mode is simplified.

In the above configuration, the booster valve may be of a normally opentype. The reduction valve may be of a normally closed type.

According to the above configuration, an opening/closing operation ofthe booster valve and the reduction valve is no longer necessary in thepseudo mode.

In the first aspect, a wheel other than the target wheel may be anon-target wheel, and in the pseudo mode, the control unit may beconfigured to reduce the brake pressure of the target wheel whilemaintaining the brake pressure of the non-target wheel, by opening thebooster valve for the target wheel, closing the reduction valve for thetarget wheel, closing the booster valve for the non-target wheel, andreducing the master pressure

In the above configuration, the control unit may be configured to set atleast one of the wheels as the target wheel and execute the antilockcontrol when a slip amount or a slip rate of the at least one of thewheels exceeds a threshold.

According to the above configuration, the brake pressure of the targetwheel can be reduced while the brake pressure of the non-target wheel ismaintained as is. In this way, the antilock control can be executed onlyfor the necessary wheels.

In the first aspect, in the pseudo mode, the control unit may beconfigured to set all the wheels as the target wheel and execute theantilock control when a locking condition is satisfied, the lockingcondition may be that at least one of the wheels shows a locking sign,and the locking sign may be that a slip amount or a slip rate exceeds athreshold.

According to the above configuration, because all the wheels are thetarget wheels, the antilock control in the pseudo mode is simplified.

In the above configuration, the locking condition may include that atleast one rear wheel shows the locking sign.

According to the above configuration, spinning can be avoided, andstability can thereby be increased.

In the above configuration, the locking condition may include that atleast one front wheel signs the locking sign.

According to the above configuration, a state where steering becomesless effective can be avoided, and the stability can thereby beincreased.

In the first aspect, the locking condition may not include that only oneof right and left front wheels shows the locking sign and the lockingcondition may further include that both of the right and left frontwheels show the locking sign.

According to the above configuration, a further large braking force canbe secured.

In the first aspect, when the vehicle travels straight, the lockingcondition may not include that only one of right and left front wheelsshows the locking sign and the locking condition may include that bothof the right and left front wheels show the locking sign, and when thevehicle turns, the locking condition may include that at least one ofthe right and left front wheels shows the locking sign.

According to the above configuration, a characteristic with a higherpriority can be obtained in accordance with a situation of the vehicle.

In the first aspect, when the vehicle travels straight, the lockingcondition may not include that only one of right and left front wheelsshows the locking sign and the locking condition may include that bothof the right and left front wheels show the locking sign, and when thevehicle turns, the locking condition may not include that only an innerwheel of the right and left front wheels shows the locking sign and thelocking condition may include that an outer wheel of the right and leftfront wheels shows the locking sign.

According to the above configuration, turnability and the braking forceduring turning can be realized in a well-balanced manner.

In the first aspect, the vehicle may include an automatic drive deviceconfigured to execute automatic drive control, and when the automaticdrive device executes track control of the vehicle, the lockingcondition may include that at least one of front wheels shows thelocking sign.

According to the above configuration, even when the antilock control inthe pseudo mode is executed, the automatic drive device can favorablyexecute the track control of the vehicle.

In the first aspect, when the vehicle travels on a split μ road, thelocking condition may not include that only the wheel on a low μ sideshows the locking sign and the locking condition may include that thewheel on a high μ side shows the locking sign.

According to the above configuration, when the vehicle travels on thesplit μ road, the further large braking force can be secured.

In the above configuration, the control unit may configured to: monitora yaw rate of the vehicle; and operate the master pressure changingdevice to reduce the master pressure when the yaw rate exceeds areference value.

According to the above configuration, the stability on the split μ roadis secured.

In the first aspect, the vehicle may include an automatic drive deviceconfigured to execute automatic drive control, and when executing theantilock control in the pseudo mode, the control unit may be configuredto request at least one of increasing a distance between vehicles,reducing a vehicle speed, notifying a driver, and prohibiting resumingof the automatic drive control to the automatic drive device.

According to the above configuration, the automatic drive control, forwhich the pseudo mode is taken into consideration, is realized.

A second aspect of the present disclosure provides a brake controlapparatus for a vehicle. The brake control apparatus according to thesecond aspect includes: a master cylinder configured to output a brakefluid at a master pressure; a master pressure changing device configuredto change the master pressure irrespective of an operation of a brakepedal; a brake actuator configured to supply the brake fluid output fromthe master cylinder to a wheel cylinder of each of wheels and control abrake pressure of the brake fluid supplied to the wheel cylinder; and acontrol unit configured to execute antilock control of preventinglocking of a target wheel by reducing the brake pressure of the targetwheel irrespective of the operation of the brake pedal during braking,the control unit being configured to execute the antilock control in apseudo mode when a pump provided in the brake actuator is unavailable,and operate the master pressure changing device such that the masterpressure obtains a target value of the brake pressure of the targetwheel, and change the brake pressure of the target wheel in aninterlocking manner with the master pressure in the pseudo mode.

According to the second aspect, even when the pump provided in the brakeactuator is unavailable, the antilock control can be executed in thepseudo mode. Thus, compared to the related art, an opportunity and aperiod to execute the antilock control are increased, and safety isimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the present disclosure will be described belowwith reference to the accompanying drawings, in which like numeralsdenote like elements, and wherein:

FIG. 1 is a schematic view of a configuration example of a vehicleaccording to an embodiment of the present disclosure;

FIG. 2 is a view of a configuration example of a brake control apparatusaccording to the embodiment of the present disclosure;

FIG. 3 is a view of a configuration example of a brake actuatoraccording to the embodiment of the present disclosure;

FIG. 4 is a timing chart that illustrates a first mode of automaticbraking control according to the embodiment of the present disclosure;

FIG. 5 is a timing chart that illustrates a second mode of the automaticbraking control according to the embodiment of the present disclosure;

FIG. 6 is a conceptual diagram that illustrates the second mode of theautomatic braking control according to the embodiment of the presentdisclosure;

FIG. 7 is a flowchart that summarizes antilock control according to theembodiment of the present disclosure;

FIG. 8 is a timing chart of examples of temporal changes in a masterpressure and a brake pressure according to the embodiment of the presentdisclosure;

FIG. 9 is a view that illustrates a modified example of the brakeactuator according to the embodiment of the present disclosure; and

FIG. 10 is a schematic view of another configuration example of thevehicle according to the embodiment of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

A description will be made on an embodiment of the present disclosurewith reference to the accompanying drawings.

1. Schematic Configuration of Vehicle

FIG. 1 is a schematic view of a configuration example of a vehicle 1according to the embodiment of the present disclosure. The vehicle 1includes wheels 10, a drive control apparatus 30, a brake controlapparatus 50, and a sensor group 70.

1-1. Wheels 10

The wheels 10 includes a front left wheel 10FL, a front right wheel10FR, a rear left wheel 10RL, and a rear right wheel 10RR.

1-2. Drive Control Apparatus 30

The drive control apparatus 30 includes an engine 31, an engineelectronic control unit (ECU) 32, a power split mechanism 33, a powertransmission mechanism 34, a generator 35, an inverter 36, a battery 37,a motor 38, and a hybrid ECU 39.

The engine 31 is a power source. The engine ECU 32 controls an operationof the engine 31. The power split mechanism 33 distributes drive powergenerated by the engine 31 to the power transmission mechanism 34 andthe generator 35. The power transmission mechanism 34 transmits thedrive power to drive wheels (the front left wheel 10FL and the frontright wheel 10FR in this example). The generator 35 generates alternatecurrent (AC) power by using the received drive power. The inverter 36converts the AC power, which is generated by the generator 35, to directcurrent (DC) power and supplies the DC power to the battery 37 to chargethe battery 37.

The motor 38 is the other power source. The inverter 36 converts the DCpower, which is discharged from the battery 37, to the AC power andsupplies the AC power to the motor 38 to execute drive control of themotor 38. Drive power generated by the motor 38 is transmitted to thedrive wheels via the power transmission mechanism 34.

The motor 38 also functions as means for generating a regenerativebraking force. More specifically, when neither the engine 31 nor themotor 38 generates the drive power during a travel of the vehicle 1,rotational forces of the drive wheels are transmitted to the motor 38via the power transmission mechanism 34. In this case, the motor 38works as a generator, and rotational resistance thereof during powergeneration serves as a braking force applied to the drive wheels. Theinverter 36 converts the AC power that is generated by the regenerativepower generation of the motor 38 to the DC power, and supplies the DCpower to the battery 37 to charge the battery 37.

The hybrid ECU 39 executes hybrid drive control that uses the two powersources of the engine 31 and the motor 38. More specifically, the hybridECU 39 generates an instruction to the engine ECU 32 and controls thedrive power generation by the engine 31. In addition, the hybrid ECU 39controls the inverter 36 to control the drive power generation by themotor 38. Furthermore, the hybrid ECU 39 controls the inverter 36 toexecute control of the regenerative brake (regenerative control).Moreover, the hybrid ECU 39 monitors a state of charge of the battery 37and controls the inverter 36 to control charging/discharging of thebattery 37.

1-3. Brake Control Apparatus 50

The brake control apparatus 50 includes a brake ECU 51, a brake pedal52, a stroke sensor 53, wheel cylinders 54FL, 54FR, 54RL, 54RR, and abrake pressure generator 55.

The brake ECU 51 is a control unit that controls an operation of thebrake control apparatus 50. The brake pedal 52 is an operation memberthat is used by a driver for a braking operation. The stroke sensor 53detects a stroke amount (an operation amount) of the brake pedal 52. Thestroke sensor 53 sends information on the detected stroke amount to thebrake ECU 51.

The wheel cylinders 54FL, 54FR, 54RL, 54RR are respectively provided inthe front left wheel 10FL, the front right wheel 10FR, the rear leftwheel 10RL, and the rear right wheel 10RR. The braking forces generatedon the front left wheel 10FL, the front right wheel 10FR, the rear leftwheel 10RL, and the rear right wheel 10RR are respectively determined bypressures of a brake fluid supplied to the wheel cylinders 54FL, 54FR,54RL, 54RR. Hereinafter, the pressures of the brake fluid supplied tothe wheel cylinders 54FL, 54FR, 54RL, 54RR will respectively be referredto as brake pressures Pfl, Pfr, Prl, Prr.

The brake pressure generator 55 supplies the brake fluid to the wheelcylinders 54FL, 54FR, 54RL, 54RR and generates the brake pressures Pfl,Pfr, Prl, Prr. The brake pressure generator 55 also has a function ofvariably controlling each of the brake pressures Pfl, Pfr, Prl, Prr.

Here, although the brake pedal 52 is coupled to the brake pressuregenerator 55, an operation of the brake pressure generator 55 does nothave to be directly correlated with an operation of the brake pedal 52.For example, a case where the above-described regenerative brake is usedwill be considered. When the driver depresses the brake pedal 52, thestroke sensor 53 sends the information on the stroke amount of the brakepedal 52 to the brake ECU 51. The brake ECU 51 computes the requestedbraking force by the driver on the basis of the stroke amount. Then, thebrake ECU 51 sends information on the requested braking force to thehybrid ECU 39. The hybrid ECU 39 executes the regenerative control onthe basis of the requested braking force to generate the regenerativebraking force. The hybrid ECU 39 sends information on the currentlygenerated regenerative braking force to the brake ECU 51. The brake ECU51 subtracts the regenerative braking force from the requested brakingforce to compute a target friction braking force. This target frictionbraking force is the braking force that should be borne by the brakecontrol apparatus 50 (the brake pressure generator 55). The brake ECU 51controls the operation of the brake pressure generator 55 so as togenerate the brake pressures Pfl, Pfr, Prl, Prr corresponding to thetarget friction braking force.

Just as described, the brake pressure generator 55 according to thisembodiment is configured to control the brake pressures Pfl, Pfr, Prl,Prr by at least following an instruction from the brake ECU 51. In otherwords, the brake pressure generator 55 can control the brake pressuresPfl, Pfr, Prl, Prr irrespective of the operation of the brake pedal 52.The brake ECU 51 can control the operation of the brake pressuregenerator 55 not only for the above-described regenerative brake butalso for various applications.

Note that the brake ECU 51 is a microcomputer including a processor,memory, and an input/output interface. The brake ECU 51 receivesdetection information from various sensors through the input/outputinterface and sends the instruction to the brake pressure generator 55.The memory stores a control program. When the processor executes thecontrol program, a function of the brake ECU 51 is realized.

1-4. Sensor Group 70

The sensor group 70 includes wheel speed sensors 71FL, 71FR, 71RL, 71RR,a steering angle sensor 72, a vehicle speed sensor 74, a lateralacceleration sensor 76, a yaw rate sensor 78, and the like. The wheelspeed sensors 71FL, 71FR, 71RL, 71RR respectively detect rotationalspeeds of the front left wheel 10FL, the front right wheel 10FR, therear left wheel 10RL, and the rear right wheel 10RR. The steering anglesensor 72 detects a steering angle. The vehicle speed sensor 74 detectsa speed of the vehicle 1. The lateral acceleration sensor 76 detectslateral acceleration (lateral G) acting on the vehicle 1. The yaw ratesensor 78 detects an actual yaw rate generated in the vehicle 1.

2. Configuration Example of Brake Control Apparatus

Hereinafter, a detailed description will be made on a configuration ofthe brake control apparatus 50, in particular, a configuration of thebrake pressure generator 55. As shown in FIG. 1, the brake pressuregenerator 55 includes a master cylinder 100, a master pressure changingdevice 200, and a brake actuator 300.

The master cylinder 100 is a brake fluid supply source. The mastercylinder 100 pushes out the brake fluid or draws the brake fluid inresponse to an external force. A pressure of the brake fluid that isoutput from the master cylinder 100 will hereinafter be referred to as a“master pressure Pm”.

The master pressure changing device 200 applies the external force tothe master cylinder 100 irrespective of the operation of the brake pedal52. When the master pressure changing device 200 increases the externalforce, the brake fluid is pushed out of the master cylinder 100, and themaster pressure Pm is boosted. On the other hand, when the masterpressure changing device 200 reduces the external force, the brake fluidis drawn into the master cylinder 100, and the master pressure Pm isreduced. That is, the master pressure changing device 200 can change themaster pressure Pm irrespective of the operation of the brake pedal 52.An operation of this master pressure changing device 200 is controlledby the brake ECU 51.

The brake actuator 300 is provided between the master cylinder 100 andthe wheel cylinders 54FL, 54FR, 54RL, 54RR. The brake actuator 300distributes the brake fluid, which is output from the master cylinder100, to the wheel cylinders 54FL, 54FR, 54RL, 54RR and generates thebrake pressures Pfl, Pfr, Prl, Prr. Each of the brake pressures Pfl,Pfr, Prl, Prr is basically changed by the master pressure Pm. However,the brake actuator 300 can individually control the brake pressures Pfl,Pfr, Prl, Prr. An operation of this brake actuator 300 is alsocontrolled by the brake ECU 51.

Hereinafter, a description will be made on specific configurationexamples of the master cylinder 100, the master pressure changing device200, and the brake actuator 300.

2-1. Master Cylinder 100

FIG. 2 shows the configuration example of the master cylinder 100. Themaster cylinder 100 includes a cylinder 110 with one opened end and theother closed end. In the following description, an opened side of thecylinder 110 will be referred to as an “A side”, and a closed sidethereof will be referred to as a “B side”.

The cylinder 110 is configured by including an input cylinder 111, apartition wall 112, an output cylinder 113, and a bottom section 114,and these are sequentially arranged from the A side to the B side. Oneend of the input cylinder 111 corresponds to an opening of the cylinder110, and the bottom section 114 corresponds to a closure of the cylinder110. The partition wall 112 is arranged between the input cylinder 111and the output cylinder 113. The partition wall 112 has a through hole112 a. An inner diameter of the through hole 112 a is smaller than aninner diameter of the input cylinder 111 and an inner diameter of theoutput cylinder 113.

An input piston 120, a first output piston 121, and a second outputpiston 122 are arranged in the cylinder 110. These input piston 120,first output piston 121, and second output piston 122 are sequentiallyarranged from the A side to the B side along an axial direction of thecylinder 110.

In detail, the input piston 120 is arranged to be slidable along aninner wall of the input cylinder 111. This input piston 120 is coupledto the brake pedal 52 via an operation rod. The input piston 120 moveslongitudinally in an interlocking manner with the operation of the brakepedal 52 by the driver.

The first output piston 121 has a projected section 121 a on the A sideand a piston section 121 b on the B side. The projected section 121 aextends from the output cylinder 113 to an inside of the input cylinder111 through the through hole 112 a of the partition wall 112. However,the projected section 121 a is not coupled to the input piston 120 andthus is not influenced by the movement of the input piston 120. As shownin FIG. 2, a separation space 130 is formed between the input piston 120and the projected section 121 a. This separation space 130 is connectedto a reservoir 132 via a passage 131.

A space surrounded by the input cylinder 111, the partition wall 112, atip of the input piston 120, and the projected section 121 a is areaction force chamber 140. The reaction force chamber 140 is filledwith the brake fluid. The reaction force chamber 140 is connected to astroke simulator 150 via a port 141 and a pipe 142.

The stroke simulator 150 includes a cylinder 151, a piston 152, a spring153, and a fluid chamber 154. One end of the cylinder 151 is opened, andthe other end thereof is closed. The piston 152 is arranged to beslidable along an inner wall of the cylinder 151. In addition, thepiston 152 is coupled to a closure of the cylinder 151 via the spring153. The fluid chamber 154 is formed between the opening of the cylinder151 and the piston 152 and is filled with the brake fluid. This fluidchamber 154 is connected to the reaction force chamber 140 via the pipe142 and the port 141.

When the driver depresses the brake pedal 52, the input piston 120 movesin a B direction. Then, the brake fluid flows from the reaction forcechamber 140 to the fluid chamber 154 of the stroke simulator 150, andthe piston 152 is pushed in a closure direction of the cylinder 151. Afluid pressure in the reaction force chamber 140 is boosted in responseto an elastic force of the spring 153 generated at the time, and thedriver receives the elastic force as a reaction force. The elastic forceof the spring 153, that is, the reaction force is proportional todisplacement of the input piston 120. It can be said that the strokesimulator 150 artificially creates a sense of the operation at a timewhen the driver depresses the brake pedal 52.

The piston section 121 b of the first output piston 121 is arranged tobe slidable along an inner wall of the output cylinder 113. A spacesurrounded by the output cylinder 113, the piston section 121 b, and thesecond output piston 122 is a first master chamber 160. The first masterchamber 160 is filled with the brake fluid. The first master chamber 160is connected to the brake actuator 300 via a port 161 and a pipe 162. Inaddition, the first master chamber 160 is connected to a reservoir 164via a port 163. A spring 165 is arranged in the first master chamber 160in a manner to connect the piston section 121 b and the second outputpiston 122.

The second output piston 122 is arranged to be slidable along the innerwall of the output cylinder 113. A space surrounded by the outputcylinder 113, the second output piston 122, and the bottom section 114of the cylinder 110 is a second master chamber 170. The second masterchamber 170 is filled with the brake fluid. The second master chamber170 is connected to the brake actuator 300 via a port 171 and a pipe172. In addition, the second master chamber 170 is connected to areservoir 174 via a port 173. A spring 175 is arranged in the secondmaster chamber 170 in a manner to connect the second output piston 122and the bottom section 114.

A state where each of the spring 165 and the spring 175 does not havethe elastic force is an initial state. FIG. 2 shows the initial state.The pressure of the brake fluid in the initial state corresponds to areservoir pressure.

As shown in FIG. 2, the piston section 121 b of the first output piston121 separates from the partition wall 112, and a servo chamber 180 isformed therebetween. The servo chamber 180 is filled with the brakefluid. A pressure of the brake fluid in the servo chamber 180 willhereinafter be referred to as a “servo pressure Ps”. The servo chamber180 is connected to the master pressure changing device 200 via a port181 and a pipe 182. The master pressure changing device 200 feeds thebrake fluid to the servo chamber 180 or draws in the brake fluid fromthe servo chamber 180.

When the master pressure changing device 200 feeds the brake fluid tothe servo chamber 180, the servo pressure Ps is boosted, and the firstoutput piston 121 moves in the B direction. When the first output piston121 moves in the B direction, communication between the first masterchamber 160 and the reservoir 164 is blocked, and the brake fluidpressure in the first master chamber 160 is boosted. As a result, thebrake fluid is output from the first master chamber 160 to the pipe 162.

Due to the boosted fluid pressure in the first master chamber 160, thesecond output piston 122 simultaneously moves in the B direction in aninterlocking manner with the first output piston 121. When the secondoutput piston 122 moves in the B direction, communication between thesecond master chamber 170 and the reservoir 174 is blocked, and thebrake fluid pressure in the second master chamber 170 is boosted. As aresult, the brake fluid is output from the second master chamber 170 tothe pipe 172.

On the other hand, when the master pressure changing device 200 draws inthe brake fluid from the servo chamber 180, the servo pressure Ps isreduced. As a result, the first output piston 121 and the second outputpiston 122 moves in an A direction, and the brake fluid pressures in thefirst master chamber 160 and the second master chamber 170 are reduced.At this time, the brake fluid is drawn into the first master chamber 160from the pipe 162, and the brake fluid is drawn into the second masterchamber 170 from the pipe 172. When the first output piston 121 and thesecond output piston 122 return to positions in the initial state, thefirst master chamber 160 is connected to the reservoir 164 again, andthe second master chamber 170 is connected to the reservoir 174 again.

Each of the pressures of the brake fluids in the first master chamber160 and the second master chamber 170 is the above-described masterpressure Pm. The master pressure Pm is substantially equal to the servopressure Ps in the servo chamber 180. The servo pressure Ps is boostedand reduced in accordance with a supply state of the brake fluid fromthe master pressure changing device 200. In other words, the masterpressure Pm can be changed by the master pressure changing device 200.

Note that the configuration of the master cylinder 100 is not limited tothe configuration shown in FIG. 2. For example, the master cylinder asdisclosed in JP 2015-136993 A may be used. In addition, when necessary,a mode in which the operation of the brake pedal 52 is directlycorrelated with a change in the master pressure Pm may be used for themaster cylinder 100.

2-2. Master Pressure Changing Device 200

FIG. 2 shows a configuration example of the master pressure changingdevice 200. The master pressure changing device 200 includes ahigh-pressure fluid source 210, a booster valve 220, a reduction valve230, a reservoir 240, and a pressure sensor 250.

The high-pressure fluid source 210 includes a hydraulic pump 211, amotor 212, a reservoir 213, an accumulator 214, and a pressure sensor215. The hydraulic pump 211 is driven by the motor 212, suctions thebrake fluid from the reservoir 213, and boosts the pressure of the brakefluid. The accumulator 214 accumulates the brake fluid, the pressure ofwhich is boosted. The pressure sensor 215 detects a pressure Ph of thebrake fluid that is accumulated in the accumulator 214. The brake ECU 51monitors the pressure Ph, which is detected by the pressure sensor 215,and controls the motor 212 such that the pressure Ph becomes equal to orhigher than a specified value.

The booster valve 220 is provided between the pipe 182, which isconnected to the servo chamber 180, and the high-pressure fluid source210. Meanwhile, the reduction valve 230 is provided between the pipe 182and the reservoir 240. The booster valve 220 is of a normally closed(NC) type, and the reduction valve 230 is of a normally open (NO) type.When the booster valve 220 is opened and the reduction valve 230 isclosed, the high-pressure brake fluid is fed to the servo chamber 180,and the servo pressure Ps and the master pressure Pm can thereby beboosted. On the other hand, when the booster valve 220 is closed and thereduction valve 230 is opened, the brake fluid is drawn from the servochamber 180, and the servo pressure Ps and the master pressure Pm canthereby be reduced.

The pressure sensor 250 detects the pressure of the brake fluid in thepipe 182, that is, the servo pressure Ps. Information on the servopressure Ps, which is detected by the pressure sensor 250, is sent tothe brake ECU 51. The brake ECU 51 controls opening/closing of thebooster valve 220 and the reduction valve 230 such that the servopressure Ps obtains a target value.

Note that the configuration of the master pressure changing device 200is not limited to the configuration thereof shown in FIG. 2. Anyconfiguration can be adopted as long as the master pressure Pm can bechanged by following the instruction from the brake ECU 51. For example,the servo pressure generator disclosed in JP 2015-136993 A may be usedas the master pressure changing device 200. Alternatively, the masterpressure Pm can be changed by using a vacuum servo device that usesnegative pressure of an engine intake system or a vacuum pump, or abooster device. In such a case, the vacuum servo device or the boosterdevice corresponds to the master pressure changing device 200.

2-3. Brake Actuator 300

FIG. 3 shows a configuration example of the brake actuator 300. Thebrake actuator 300 includes input nodes 310F, 310R, valve units 320FL,320FR, 320RL, 320RR, reservoirs 330F, 330R, and a pump unit 350.

The input node 310F is connected to the second master chamber 170 of themaster cylinder 100 via the pipe 172. The input node 310F receives thebrake fluid that is output from the second master chamber 170. The inputnode 310R is connected to the first master chamber 160 of the mastercylinder 100 via the pipe 162. The input node 310R receives the brakefluid that is output from the first master chamber 160.

The valve units 320FL, 320FR, 320RL, 320RR are respectively provided forthe wheel cylinders 54FL, 54FR, 54RL, 54RR. More specifically, the valveunit 320FL is provided between the input node 310F and the wheelcylinder 54FL. The valve unit 320FR is provided between the input node310F and the wheel cylinder 54FR. The valve unit 320RL is providedbetween the input node 310R and the wheel cylinder 54RL. The valve unit320RR is provided between the input node 310R and the wheel cylinder54RR.

Each of the valve units 320FL, 320FR, 320RL, 320RR includes a boostervalve 321, a reduction valve 322, and a check valve 323. The boostervalve 321 and the reduction valve 322 are electromagnetic valves(solenoid valves), for example.

A description will representatively be made on the valve unit 320FL. Thebooster valve 321 is provided between the input node 310F and the wheelcylinder 54FL. The reduction valve 322 is provided between the wheelcylinder 54FL and the reservoir 330F. For example, the booster valve 321is of the NO type, and the reduction valve 322 is of the NC type. Thecheck valve 323 is connected in a manner to only permit a flow of thebrake fluid from the wheel cylinder 54FL to the input node 310F. In thecase where the master pressure Pm becomes lower than a brake pressurePfl when the booster valve 321 is closed, the brake fluid flows throughthe check valve 323, and the brake pressure Pfl is reduced.

The brake ECU 51 can variably control the brake pressure Pfl of thewheel cylinder 54FL by controlling an operation of such a valve unit320FL. More specifically, the brake ECU 51 can boost the brake pressurePfl within a range that is equal to or lower than the master pressure Pmby opening the booster valve 321 and closing the reduction valve 322. Onthe other hand, the brake ECU 51 causes the brake fluid to flow from thewheel cylinder 54FL to the reservoir 330F by closing the booster valve321 and opening the reduction valve 322, and can thereby reduce thebrake pressure Pfl. The brake ECU 51 can variably control the brakepressure Pfl by controlling opening/closing of the booster valve 321 andthe reduction valve 322.

The same applies to the other valve units 320FR, 320RL, 320RR. In casesof the valve units 320RL, 320RR, the “input node 310F” is switched tothe “input node 310R”, and the “reservoir 330F” is switched to the“reservoir 330R”.

The pump unit 350 is configured to cause the brake fluid to return fromthe reservoirs 330F, 330R to the input nodes 310F, 310R, respectively,by following the instruction from the brake ECU 51. In detail, the pumpunit 350 includes pumps 351F, 351R, check valves 352F, 352R, checkvalves 353F, 353R, and a motor unit 355.

The pump 351F is provided between the reservoir 330F and the input node310F, and is configured to cause the brake fluid to return from thereservoir 330F to the input node 310F. The check valve 352F is connectedin a manner to only permit a flow of the brake fluid from the reservoir330F to the pump 351F. The check valve 353F is connected in a manner toonly permit a flow of the brake fluid from the pump 351F to the inputnode 310F. This check valve 353F prevents application of thehigh-pressure brake fluid to the pump 351F.

Similarly, the pump 351R is provided between the reservoir 330R and theinput node 310R, and is configured to cause the brake fluid to returnfrom the reservoir 330R to the input node 310R. The check valve 352R isconnected in a manner to only permit a flow of the brake fluid from thereservoir 330R to the pump 351R. The check valve 353R is connected in amanner to only permit a flow of the brake fluid from the pump 351R tothe input node 310R. This check valve 353R prevents the application ofthe high-pressure brake fluid to the pump 351R.

The motor unit 355 executes drive control of the pumps 351F, 351R. Morespecifically, the motor unit 355 includes: a motor that drives the pumps351F, 351R; and a motor controller that controls an operation of themotor. The motor controller receives a drive command from the brake ECU51, and issues a motor drive command that corresponds to the receiveddrive command. Then, the motor controller supplies the motor drivecommand to the motor and thereby drives the motor and the pumps 351F,351R. By driving the pumps 351F, 351R, the brake fluid can return fromthe reservoirs 330F, 330R to the input nodes 310F, 310R, respectively.

The pump unit 350 has a self-detection function of detecting that thepumps 351F, 351R are not actuated normally. When detecting that thepumps 351F, 351R are not actuated normally, the pump unit 350 outputs anerror signal to the brake ECU 51. For example, when the motor fails, thepumps 351F, 351R are not actuated normally. The motor controllerreceives motor rotational state information from a sensor attached tothe motor, compares the motor drive command and the motor rotationalstate information, and can thereby determine whether the motor isnormally actuated. When the motor is not actuated normally, the motorcontroller outputs the error signal to the brake ECU 51.

Note that the configuration of the brake actuator 300 is not limited tothe configuration shown in FIG. 3. Any configuration can be adopted aslong as the brake actuator 300 can individually control the brakepressures Pfl, Pfr, Prl, Prr by following the instruction from the brakeECU 51. For example, such a configuration may be adopted that a mastercut valve is provided between each of the input nodes 310F, 310R and themaster cylinder 100 and the brake pressures Pfl, Pfr, Prl, Prr can beboosted by driving the pumps 351F, 351R.

3. Two Modes of Automatic Braking Control

The brake ECU 51 executes “automatic braking control” in which thebraking force is changed irrespective of the operation of the brakepedal 52. According to this embodiment, as modes of the automaticbraking control, two types of a “first mode” and a “second mode” of theautomatic braking control are provided. Hereinafter, a detaileddescription will be made on each of the first mode and the second mode.

Note that, in the following description, brake pressures of the frontleft wheel 10FL, the front right wheel 10FR, the rear left wheel 10RL,and the rear right wheel 10RR will respectively mean the brake pressuresPfl, Pfr, Prl, Prr of the wheel cylinders 54FL, 54FR, 54RL, 54RR. Inaddition, valve units for the front left wheel 10FL, the front rightwheel 10FR, the rear left wheel 10RL, and the rear right wheel 10RR willrespectively mean the valve units 320FL, 320FR, 320RL, 320RR, which arerespectively connected to the wheel cylinders 54FL, 54FR, 54RL, 54RR.

Of the front left wheel 10FL, the front right wheel 10FR, the rear leftwheel 10RL, and the rear right wheel 10RR, the wheel, the brake pressureof which is changed, will be referred to as a “target wheel 10T”. Thevalve unit for the target wheel 10T will be referred to as a “valve unit320T”. The brake pressure of the target wheel 10T will be referred to asa “brake pressure Pt”. The wheel other than the target wheel 10T will bereferred to as a “non-target wheel 10NT”. The valve unit for thenon-target wheel 10NT will be referred to as a “valve unit 320NT”. Thebrake pressure of the non-target wheels 10NT will be referred to as a“brake pressure Pnt”.

3-1. First Mode

FIG. 4 is a timing chart that shows the master pressure Pm and the brakepressure Pt of the target wheel 10T in the first mode. A horizontal axisrepresents time, and a vertical axis represents pressure. The automaticbraking control for the target wheel 10T is executed in a period fromtime is to time te.

The brake ECU 51 operates the master pressure changing device 200 andboosts the master pressure Pm to certain extent. Meanwhile, based on thedetection information received from the sensor group 70, the brake ECU51 computes a target value of the brake pressure Pt of the target wheel10T that is required for the automatic braking control. The target valuewill hereinafter be referred to as a “target brake pressure”. The brakeECU 51 operates the brake actuator 300 in a manner to obtain the targetbrake pressure.

More specifically, in regard to the non-target wheel 10NT, the brake ECU51 closes the booster valve 321 and the reduction valve 322 of the valveunit 320NT. In this way, the brake pressure Pnt of the non-target wheel10NT is not influenced by the master pressure Pm and is maintained asis. Meanwhile, in regard to the target wheel 10T, the brake ECU 51controls an operation of the valve unit 320T in a manner to obtain thetarget brake pressure. More specifically, the brake pressure Pt can beboosted by opening the booster valve 321 and closing the reduction valve322. On the other hand, the brake pressure Pt can be reduced by closingthe booster valve 321 and opening the reduction valve 322. Bycontrolling opening/closing of the booster valve 321 and the reductionvalve 322, the brake pressure Pt can be controlled at the target brakepressure.

As shown in FIG. 4, the first mode has a characteristic that the brakepressure Pt of the target wheel 10T is not changed in an interlockingmanner with the master pressure Pm. That is, in the first mode, thebrake ECU 51 changes the brake pressure Pt of the target wheel 10T in amanner not to interlock with the master pressure Pm by operating thebrake actuator 300.

3-2. Second Mode

FIG. 5 is a timing chart that shows the master pressure Pm and the brakepressure Pt of the target wheel 10T in the second mode. A horizontalaxis and a vertical axis of FIG. 5 represent the same as those in FIG.4. Differing from the first mode, the second mode has a characteristicthat the brake pressure Pt of the target wheel 10T is changed in aninterlocking manner with the master pressure Pm.

A description will be made on a method for realizing the second modewith reference to FIG. 6. In regard to the target wheel 10T, the brakeECU 51 opens the booster valve 321 of the valve unit 320T and closes thereduction valve 322 of the valve unit 320T. When the booster valve 321is of the NO type and the reduction valve 322 is of the NC type, thebrake ECU 51 does not have to operate the valve unit 320T for the targetwheel 10T. Meanwhile, in regard to the non-target wheel 10NT, the brakeECU 51 closes the booster valve 321 and the reduction valve 322 of thevalve unit 320NT.

Based on the detection information received from the sensor group 70,the brake ECU 51 computes the target brake pressure of the target wheel10T that is required for the automatic braking control. Then, the brakeECU 51 operates the master pressure changing device 200 such that themaster pressure Pm obtains the target brake pressure. That is, the brakeECU 51 changes the master pressure Pm in a similar manner to the targetbrake pressure. At this time, as shown in FIG. 6, the brake pressure Pt,which is substantially equal to the master pressure Pm, is applied tothe target wheel 10T through the valve unit 320T. As a result, the brakepressure Pt of the target wheel 10T is changed in the interlockingmanner with the master pressure Pm. Meanwhile, the brake pressure Pnt ofthe non-target wheel 10NT is not changed and is maintained as is.

When the plural target wheels are present, the target brake pressurepossibly differs by the target wheel. In this case, the brake ECU 51operates the master pressure changing device 200 such that the masterpressure Pm obtains a maximum value of the target brake pressure. Also,in this case, the brake pressure Pt of any of the target wheels 10T ischanged in the interlocking manner with the master pressure Pm at eachtime point.

Just as described, the second mode has a characteristic that the brakepressure Pt of the target wheel 10T is changed in the interlockingmanner with the master pressure Pm. That is, in the second mode, thebrake ECU 51 changes the master pressure Pm by operating the masterpressure changing device 200, and changes the brake pressure Pt of thetarget wheel 10T in a manner to interlock with the master pressure Pm.

4. Antilock Control

“Antilock control” as one type of the automatic braking control will beconsidered. The antilock control is the automatic braking control forpreventing locking of the wheel during braking and is also referred toas antilock brake system (ABS) control.

The brake ECU 51 executes the antilock control by reducing the brakepressure Pt of the target wheel 10T. As described above, as means forchanging the brake pressure Pt of the target wheel 10T, the two types ofthe first mode and the second mode are available. According to thisembodiment, the brake ECU 51 uses the first mode or the second modedepending on whether the first mode is available.

As one example, a case where the pump unit 350 of the brake actuator 300shown in FIG. 3 is not actuated normally will be considered. In thiscase, the pumps 351F, 351R are not actuated normally, and thus the brakefluid cannot return from the reservoir 330F, 330R to the input nodes310F, 310R, respectively. Accordingly, after the reservoirs 330F, 330Rbecome full, the brake pressures Pfl, Pfr, Prl, Prr cannot be reduced.This means that the antilock control in the first mode cannot beexecuted normally, that is, the first mode is unavailable.

For this reason, according to this embodiment, when detecting that thepump unit 350 is not actuated normally, the brake ECU 51 prohibits thefirst mode and permits the second mode. For example, the pump unit 350has the self-detection function of detecting that the pumps 351F, 351Rare not actuated normally. When detecting that the pumps 351F, 351R arenot actuated normally, the pump unit 350 outputs an error signal to thebrake ECU 51. In response to the error signal, the brake ECU 51prohibits the first mode and permits the second mode.

As another example, the brake ECU 51 may prohibit the first mode andpermit the second mode in response to a request from a system.

From a perspective of responsiveness of the brake pressures Pfl, Pfr,Prl, Prr, the first mode is superior to the second mode. Thus, when thefirst mode is available, as usual, the brake ECU 51 executes theantilock control in the first mode. In this sense, the first mode can bereferred to as a “normal mode”. Meanwhile, the second mode can bereferred to as a “pseudo mode” or an “emergency mode”.

FIG. 7 is a flowchart that summarizes antilock control according to thisembodiment. The brake ECU 51 repeatedly executes a processing flow shownin FIG. 7.

Step S10: The brake ECU 51 receives the detection information from thesensor group 70 to comprehend a travel state of the vehicle 1. Then,based on the travel state, the brake ECU 51 determines whether a“locking condition” is satisfied. Typically, the locking condition isthat a slip amount or a slip rate of at least one wheel exceeds athreshold. If the locking condition is satisfied (step S10; Yes), theprocessing proceeds to step S20. On the other hand, if the lockingcondition is not satisfied (step S10; No), the processing in the currentcycle is terminated.

Step S20: The brake ECU 51 executes the antilock control. At this time,the brake ECU 51 checks whether the first mode is available (step S21).If the first mode is permitted, that is, if the first mode is available,the brake ECU 51 executes the antilock control in the first mode (stepS22). On the other hand, if the first mode is prohibited, that is, ifthe first mode is unavailable, the brake ECU 51 executes the antilockcontrol in the second mode (step S23).

As it has been described so far, according to this embodiment, even whenthe first mode (the normal mode) is unavailable, the antilock controlcan be executed in the second mode (the pseudo mode). Thus, compared tothe related art, an opportunity and a period to execute the antilockcontrol are increased, and safety is improved.

It can also be said that this embodiment uses “redundancy” that both ofthe first mode and the second mode are present. That is, when the firstmode is unavailable, the brake ECU 51 does not give up the execution ofthe antilock control and, instead of the first mode, can execute theantilock control in the second mode. This embodiment is particularlyeffective in such a situation where the antilock control cannot easilybe abandoned (for example, during self-driving).

A case where the first mode becomes unavailable during self-driving ofthe vehicle 1 will be considered. Also in this case, the antilockcontrol does not become void, and the antilock control in the secondmode is effective. In this way, the safety is maintained, and thusself-driving can be continued. That is, there is no need to stopself-driving simply because the first mode is no longer available.According to this embodiment, continuity of self-driving can beimproved.

5. Specific Example of Antilock Control in Pseudo Mode

Hereinafter, a detailed description will be made on the antilock controlin the pseudo mode (the second mode). First, terms that will be used inthe following description are defined. When a certain wheel shows a“lock sign”, it means that the slip amount or the slip rate of saidwheel exceeds the threshold. Based on the rotational speed of each ofthe wheels and the speed of the vehicle 1, the brake ECU 51 can computethe slip amount or the slip rate of each of the wheels. The rotationalspeeds of the wheels are respectively detected by the wheel speedsensors 71FL, 71FR, 71RL, 71RR. The speed of the vehicle 1 is detectedby the vehicle speed sensor 74. Alternatively, the speed of the vehicle1 may be computed from the rotational speeds of the front left wheel10FL, the front right wheel 10FR, the rear left wheel 10RL, and the rearright wheel 10RR. The brake ECU 51 can determine whether the lockingsign appears per wheel.

As shown in FIG. 7, if the locking condition is satisfied (step S10;Yes), the brake ECU 51 executes the antilock control. Typically, thelocking condition is that the at least one wheel shows the locking sign.The brake ECU 51 at least sets the wheel that shows the locking sign asthe target wheel 10T, and executes the antilock control.

As the simplest mode, it is considered to set all the wheels as thetarget wheels 10T. In this case, in regard to all the wheels, the brakeECU 51 opens the booster valves 321 and closes the reduction valves 322.When each of the booster valves 321 is of the NO type and each of thereduction valves 322 is of the NC type, the brake ECU 51 does not haveto perform opening/closing operations of the valves and thus isfavorable. Then, the brake ECU 51 reduces the brake pressures Pfl, Pfr,Prl, Prr of all the wheels by reducing the master pressure Pm.

FIG. 8 is a timing chart that shows one example of temporal changes inthe master pressure Pm and the brake pressures Pfl, Pfr, Prl, Prr. Ahorizontal axis represents time, and a vertical axis representspressure. FIG. 8 also shows a tendency of a wheel speed of each of thefront left wheel 10FL, the front right wheel 10FR, the rear left wheel10RL, and the rear right wheel 10RR.

At time to, the brake ECU 51 initiates normal braking control. The brakeECU 51 computes a target master pressure that corresponds to the targetfriction braking force, and boosts the master pressure Pm to the targetmaster pressure. Corresponding to the boosted master pressure Pm, thebrake pressures Pfl, Pfr, Prl, Prr are also boosted. In this way, thebraking force is generated on each of the wheels, and the speed of thevehicle 1 is thereby reduced.

During braking, the rear right wheel 10RR shows the locking sign. Thebrake ECU 51 detects the locking sign of the right rear wheel 10RR andinitiates the antilock control at time ts. More specifically, the brakeECU 51 reduces the master pressure Pm. Corresponding to the reducedmaster pressure Pm, all the brake pressures Pfl, Pfr, Prl, Prr are alsoreduced. The locking sign of the right rear wheel 10RR disappears whenat least the brake pressure Prr is reduced.

The brake ECU 51 detects that the locking sign of the right rear wheel10RR has disappeared, and terminates the antilock control at the timete. Then, the brake ECU 51 returns the master pressure Pm to theoriginal master pressure Pm, which also returns the brake pressures Pfl,Pfr, Prl, Prr to the original brake pressures Pfl, Pfr, Prl, Prr.

Note that the present disclosure of the subject application is notlimited to a case where all the wheels are set as the target wheels 10T.The wheel that shows the locking sign may be set as the target wheel10T, and the wheels other than this may be set as the non-target wheels10NT. In this case, as shown in FIG. 6, which has already beendescribed, the brake ECU 51 closes the booster valve 321 of thenon-target wheel 10NT. However, due to presence of the check valve 323,when the master pressure Pm is reduced to be lower than the brakepressure Pnt, the brake fluid flows through the check valve 323, andconsequently the brake pressure Pnt is also reduced. In order not tochange the brake pressure Pnt of the non-target wheel 10NT and tomaintain the brake pressure Pnt as is in the pseudo mode, it isnecessary to invalidate the check valve 323.

For example, as shown in FIG. 9, an invalidation valve 324 of the NOtype that invalidates the check valve 323 is provided. When the antilockcontrol in the pseudo mode is executed, the brake ECU 51 closes thebooster valve 321 and the invalidation valve 324 for the non-targetwheel 10NT. In this way, even when the master pressure Pm is reduced,the brake pressure Pnt of the non-target wheel 10NT is not changed andcan be maintained as is. The brake ECU 51 can reduce the brake pressurePt of the target wheel 10T only while maintaining the brake pressure Pntof the non-target wheel 10NT as is.

A description will hereinafter be made on various examples of the“locking condition” that triggers initiation of the antilock control.

5-1. First Example

When either one of the rear wheels 10RL, 10RR is locked, an oversteertendency is increased, and the vehicle 1 is likely to spin. Accordingly,when at least one of the rear wheels 10RL, 10RR shows the locking sign,the brake ECU 51 executes the antilock control. In this way, spinningcan be avoided, and the stability can thereby be increased.

In addition, when either one of the front wheels 10FL, 10FR is locked,steering becomes less effective. Accordingly, when at least one of thefront wheels 10FL, 10FR shows the locking sign, the brake ECU 51executes the antilock control. In this way, a state where steeringbecomes less effective can be avoided, and the stability can thereby beincreased.

5-2. Second Example

In a second example, a case where all the wheels are the target wheels10T will be considered. In the cases where all the wheels are the targetwheels 10T and the master pressure Pm is reduced, the brake pressure ofthe wheel that does not show the locking sign is also reduced. As aresult, the braking force is reduced by the reduction in the brakepressure just as described. In view of the above, in the second example,the locking condition for the front wheels 10FL, 10FR is more relaxedthan that in the first example.

More specifically, at a stage where only one of the front wheels 10FL,10FR shows the locking sign, the brake ECU 51 does not execute theantilock control. When both of the front wheels 10FL, 10FR show thelocking sign, the brake ECU 51 executes the antilock control. That is,as the locking condition, a condition that only one of the front wheels10FL, 10FR shows the locking sign is not included, but a condition thatboth of the front wheels 10FL, 10FR show the locking sign is included.In this way, the opportunity of the antilock control, which is caused bythe locking signs of the front wheels 10FL, 10FR, is reduced. As aresult, the further large braking force can be secured.

The locking condition for the rear wheels 10RL, 10RR is the same as thatin the first example. That is, when at least one of the rear wheels10RL, 10RR shows the locking sign, the brake ECU 51 executes theantilock control regardless of the states of the front wheels 10FL,10FR. In this way, spinning can be avoided. It can be said that thebraking force is secured to be as large as possible while spinning isavoided in the second example.

5-3. Third Example

A third example is a modified example of the second example. In thethird example, the brake ECU 51 changes the locking condition related tothe front wheels 10FL, 10FR in accordance with whether the vehicle 1travels straight or turns. Note that whether the vehicle 1 travelsstraight or turns can be determined on the basis of the detectioninformation from the steering angle sensor 72, the lateral accelerationsensor 76, and the yaw rate sensor 78.

When the vehicle 1 travels straight, the braking force takes precedenceover steerability. Thus, the locking condition is set to be the same asthat in the second example. That is, as the locking condition, thecondition that only one of the front wheels 10FL, 10FR shows the lockingsign is not included, but the condition that both of the front wheels10FL, 10FR show the locking sign is included.

Meanwhile, when the vehicle 1 turns, the steerability and turnabilitytake precedence. Thus, the locking condition is set to be the same asthat in the first example. Typically, the locking condition includesthat the at least one of the front wheels 10FL, 10FR shows the lockingsign.

Just as described, according to the third example, the locking conditionfor the front wheels 10FL, 10FR is set in accordance with a situation ofthe vehicle 1. In this way, a characteristic with a higher priority canbe obtained in accordance with the situation of the vehicle 1.

5-4. Fourth Example

A fourth example is a further modified example of the third example. Thelocking condition of the case where the vehicle 1 travels straight isthe same as that in the third example. Meanwhile, the locking conditionof the case where the vehicle 1 turns differs by whether the wheel thatshows the locking sign is an inner front wheel or an outer front wheel.

During turning, a larger load is applied to the outer front wheel thanto the inner front wheel. When the outer front wheel is locked, alateral force is significantly lost. Accordingly, when the outer frontwheel shows a locking tendency, the brake ECU 51 executes the antilockcontrol to maintain the turnability. Meanwhile, when only the innerfront wheel shows the locking tendency, the braking force takesprecedence, and the antilock control is not executed. That is, when thevehicle 1 turns, the locking condition does not include a condition thatonly the inner front wheel shows the locking sign, but includes acondition that the outer front wheel shows the locking sign.

Just as described, according to the fourth example, the turnability andthe braking force during turning can be realized in a well-balancedmanner.

5-5. Fifth Example

In a fifth example, a split μ road will be considered. On the split μroad, a friction coefficient (μ) differs between a side of the leftwheels 10FL, 10RL and a side of the right wheels 10FR, 10RR. The brakeECU 51 can determine whether the vehicle 1 travels on the split μ roadby contrasting the slip amount or the slip rate of the left wheels 10FL,10FR and that of the right wheels 10FR, 10RR.

When the vehicle 1 travels on the split μ road, the wheel on the low μside is likely to be locked. However, in the cases where all the wheelsare the target wheels 10T and the master pressure Pm is reduced, thebrake pressure on the high μ side is also reduced. As a result, thebraking force is reduced by the reduction in the brake pressure just asdescribed. In view of the above, in the fifth example, in order tosecure the larger braking force as possible, the locking condition onthe low μ side is more relaxed than that in the first example.

More specifically, at a stage where only the wheels on the low μ sideshow the locking sign, the brake ECU 51 does not execute the antilockcontrol. When at least one of the wheels on the high μ side shows thelocking sign, the brake ECU 51 executes the antilock control. That is,the locking condition does not include a condition that only the wheelson the low μ side show the locking sign, but includes a condition thatthe wheel on the high μ side shows the locking sign. In this way, thefurther large braking force can be secured.

Note that, when a difference in the braking force between the leftwheels 10FL, 10RL and the right wheels 10FR, 10RR is increased, the yawrate is increased, and the vehicle 1 is likely to spin. Accordingly, thebrake ECU 51 monitors the yaw rate on the basis of the detectioninformation from the yaw rate sensor 78. When the yaw rate exceeds areference value, the brake ECU 51 operates the master pressure changingdevice 200 to reduce the master pressure Pm. In this way, the brakepressures of all the wheels as the target wheels 10T are also reduced.When the brake pressures are reduced, the difference in the brakingforce between the left wheels 10FL, 10RL and the right wheels 10FR, 10RRis reduced. As a result, the yaw rate is reduced, and the stability onthe split μ road is secured.

6. Cooperation with Automatic Drive Device

FIG. 10 schematically shows a configuration of the vehicle 1 on which anautomatic drive device 90 is mounted. The automatic drive device 90executes automatic drive control. For example, the automatic drivedevice 90 executes track control that makes the vehicle 1 automaticallytravel along a travel plan.

When the automatic drive device 90 executes the track control, it ispreferred to secure the steerability so as to prevent deviation of thevehicle 1 from a travel plan route. Accordingly, as the lockingcondition of a case where the automatic drive device 90 executes thetrack control, the above-described first example is preferred. That is,when at least one of the front wheels 10FL, 10FR shows the locking sign,the brake ECU 51 executes the antilock control. In this way, thesteerability is secured, and the track control can favorably beexecuted.

From a perspective of responsiveness of the brake pressure during theantilock control, the normal mode is superior to the pseudo mode. Inaddition, in the pseudo mode, the braking force in the case where allthe wheels are the target wheels 10T is smaller than that in the normalmode. From what has been described so far, in the case of the pseudomode, a time required to avoid a collision is possibly extended from thecase of the normal mode.

In view of the above, when executing antilock control in the pseudomode, the brake ECU 51 informs the automatic drive device 90 of “use ofthe pseudo mode” and requests at least one of the following (1) to (4)to the automatic drive device 90. (1) Increase a distance betweenvehicles: the collision can easily be avoided by increasing the distancebetween the vehicles in advance. (2) Reduce the vehicle speed: thecollision can easily be avoided by reducing the vehicle speed inadvance. (3) Notify the driver: when being notified of the use of thepseudo mode, the driver can recognize a cause of a driving sense thatdiffers from that in the case of the normal mode, and thus can feelsafe. (4) Prohibit resuming of the automatic drive control: theautomatic drive device 90 continues the automatic drive control of thecurrent time; however, once stopping the automatic drive control, theautomatic drive device 90 does not resume the automatic drive control.In this way, a risk of not being able to use the normal mode is avoided.

Just as described, according to this embodiment, the automatic drivecontrol, for which the pseudo mode is taken into consideration, isrealized.

What is claimed is:
 1. A brake control apparatus for a vehiclecomprising: a master cylinder configured to output a brake fluid at amaster pressure; a master pressure changing device configured to changethe master pressure irrespective of an operation of a brake pedal; abrake actuator configured to supply the brake fluid output from themaster cylinder to a wheel cylinder of each of wheels and that cancontrol a brake pressure of the brake fluid supplied to the wheelcylinder; and a control unit configured to execute antilock control ofpreventing locking of a target wheel by reducing the brake pressure ofthe target wheel irrespective of the operation of the brake pedal duringbraking, wherein modes of the antilock control include a normal mode anda pseudo mode, and the control unit is configured to in the normal mode,operate the brake actuator to obtain a target value of the brakepressure of the target wheel, in the pseudo mode, operate the masterpressure changing device such that the master pressure obtains thetarget value, and change the brake pressure of the target wheel suchthat the brake pressure of the target wheel and the mater pressure aresubstantially a same pressure over time, and execute the antilockcontrol in the pseudo mode when the normal mode is unavailable.
 2. Thebrake control apparatus for the vehicle according to claim 1 wherein thebrake actuator includes a pump, and the control unit is configured toexecute the antilock control in the pseudo mode when the pump cannot beused.
 3. The brake control apparatus for the vehicle according to claim1 wherein the brake actuator includes: an input node that receives thebrake fluid output from the master cylinder; a booster valve that isprovided between the input node and the wheel cylinder for each of thewheels; a reduction valve that is provided between the wheel cylinderand a reservoir for each of the wheels; and a pump that is configured tocauses the brake fluid to return from the reservoir to the input node.4. The brake control apparatus for the vehicle according to claim 3wherein in the pseudo mode, all the wheels are the target wheels, andthe control unit is configured to reduce the brake pressures by openingthe booster valves and closing the reduction valves for all the wheelsand reducing the master pressure.
 5. The brake control apparatus for thevehicle according to claim 4 wherein the booster valve is of a normallyopen type, and the reduction valve is of a normally closed type.
 6. Thebrake control apparatus for the vehicle according to claim 3 wherein awheel other than the target wheel is a non-target wheel, and in thepseudo mode, the control unit is configured to reduce the brake pressureof the target wheel while maintaining the brake pressure of thenon-target wheel, by opening the booster valve for the target wheel,closing the reduction valve for the target wheel, closing the boostervalve for the non-target wheel, and reducing the master pressure.
 7. Thebrake control apparatus for the vehicle according to claim 6 wherein thecontrol unit is configured to set at least one of the wheels as thetarget wheel and execute the antilock control when a slip amount or aslip rate of the at least one of the wheels exceeds a threshold.
 8. Thebrake control apparatus for the vehicle according to claim 1 wherein inthe pseudo mode, the control unit is configured to set all the wheels asthe target wheel and execute the antilock control when a lockingcondition is satisfied, the locking condition is that at least one ofthe wheels shows a locking sign, and the locking sign is that a slipamount or a slip rate exceeds a threshold.
 9. The brake controlapparatus for the vehicle according to claim 8 wherein the lockingcondition includes that at least one rear wheel shows the locking sign.10. The brake control apparatus according to claim 9 wherein the lockingcondition includes that at least one front wheel shows the locking sign.11. The brake control apparatus for the vehicle according to claim 9wherein the locking condition does not include that only one of rightand left front wheels shows the locking sign and the locking conditionfurther includes that both of the right and left front wheels show thelocking sign.
 12. The brake control apparatus for the vehicle accordingto claim 9 wherein when the vehicle travels straight, the lockingcondition does not include that only one of right and left front wheelsshows the locking sign and the locking condition includes that both ofthe right and left front wheels show the locking sign, and when thevehicle turns, the locking condition includes that at least one of theright and left front wheels shows the locking sign.
 13. The brakecontrol apparatus for the vehicle according to claim 9 wherein when thevehicle travels straight, the locking condition does not include thatonly one of right and left front wheels shows the locking sign and thelocking condition includes that both of the right and left front wheelsshow the locking sign, and when the vehicle turns, the locking conditiondoes not include that only an inner wheel of the right and left frontwheels shows the locking sign and the locking condition includes that anouter wheel of the right and left front wheels shows the locking sign.14. The brake control apparatus for the vehicle according to claim 9wherein the vehicle includes an automatic drive device configured toexecute automatic drive control, and when the automatic drive deviceexecutes track control of the vehicle, the locking condition includesthat at least one of front wheels shows the locking sign.
 15. The brakecontrol apparatus for the vehicle according to claim 8 wherein when thevehicle travels on a split μ road, the locking condition does notinclude that only the wheel on a low μ side shows the locking sign andthe locking condition includes that the wheel on a high μ side shows thelocking sign.
 16. The brake control apparatus for the vehicle accordingto claim 15 wherein the control unit is configured to: monitor a yawrate of the vehicle; and operate the master pressure changing device toreduce the master pressure when the yaw rate exceeds a reference value.17. The brake control apparatus for the vehicle according to claim 1wherein the vehicle includes an automatic drive device configured toexecute automatic drive control, and when executing the antilock controlin the pseudo mode, the control unit is configured to request at leastone of increasing a distance between vehicles, reducing a vehicle speed,notifying a driver, and prohibiting resuming of the automatic drivecontrol to the automatic drive device.
 18. A brake control apparatus fora vehicle comprising: a master cylinder configured to output a brakefluid at a master pressure; a master pressure changing device configuredto change the master pressure irrespective of an operation of a brakepedal; a brake actuator configured to supply the brake fluid output fromthe master cylinder to a wheel cylinder of each of wheels and control abrake pressure of the brake fluid supplied to the wheel cylinder; and acontrol unit configured to execute antilock control of preventinglocking of a target wheel by reducing the brake pressure of the targetwheel irrespective of the operation of the brake pedal during braking,the control unit being configured to execute the antilock control in apseudo mode when a pump provided in the brake actuator is unavailable,and operate the master pressure changing device such that the masterpressure obtains a target value of the brake pressure of the targetwheel, and change the brake pressure of the target wheel such that thebrake pressure of the target wheel and the mater pressure aresubstantially a same pressure over time, with the master pressure in thepseudo mode.